Mitigation of Harmonic Currents and Conservation of Power in Non-Linear Load Systems

ABSTRACT

An AC power controller system applies three-phase AC operating power to an induction motor that drives a non-linear mechanical load. A primary low pass filter is connected in series between branch phase conductors and a power controller of the type that uses gate-controlled switching thyristors for controlling power to the motor. KVAR capacitors connected between the power controller and the induction motor phase windings form a secondary low pass filter across the controller output terminals. The primary and secondary low pass filters isolate the power controller and induction motor with respect to spurious noise and harmonics generated by local as well as remote sources, and also improve real power transfer efficiency from the power generating source to the induction motor by transforming the effective impedance of the power source and induction motor load.

PRIORITY STATEMENT & CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of co-pending U.S. patent applicationSer. No. 12/577,479, entitled “Mitigation of Harmonic Currents andConservation of Power in Non-Linear Load Systems” and filed on Oct. 12,2009, in the name of Filiberto D. Garza; which is a continuation of U.S.patent application Ser. No. 11/959,411, entitled “Mitigation of HarmonicCurrents and Conservation of Power in Non-Linear Load Systems” filed onSep. 18, 2007 and issued on Oct. 13, 2009 as U.S. Pat. No. 7,602,136 inthe name of Filiberto D. Garza; which is continuation of U.S. patentapplication Ser. No. 11/379,965, entitled “Mitigation of HarmonicCurrents and Conservation of Power in Non-Linear Load Systems,” filed onApr. 24, 2006 and issued on Dec. 18, 2007 as U.S. Pat. No. 7,309,973 inthe name of Filiberto D. Garza; both of which are hereby incorporated byreference for all purposes.

This invention is related to the subject matter of U.S. Pat. No.6,400,119 entitled “Energy Conserving Motor Controller,” which isassigned to the assignee of the present invention and incorporatedherein by reference for all purposes.

TECHNICAL FIELD OF THE INVENTION

This invention is related generally to AC power distribution systems,and in particular to AC power controller systems that control theapplication of AC operating power to induction motors.

BACKGROUND OF THE INVENTION

Spurious noise signals, including harmonic currents, background noiseand spike impulse noise are developed on AC power distribution lines.Such noise signals can originate from the power source, the distributionnetwork, local and remote loads coupled to the network, lightningstrikes and distribution equipment malfunction. The AC supply currentdelivered from a public utility is not a pure sine wave and containsharmonics that interfere with proper operation of connected equipment.Additionally, noise and switching transients may be introduced fromactive loads. By way of example, if a branch is loaded by an electronicdimmer and lamp, the dimmer will “chop” the 60 Hz AC power waveform at ahigh frequency to reduce the lighting intensity. This will introduceharmonics and high frequency noise on the power distribution conductors.

Such noise is not constant with respect to time, and it also varies fromplace to place in the power distribution network. Moreover, a typical ACpower line network distributes power to a variety of electrical loaddevices. Each load can conduct a significant level of noise and harmoniccurrents back onto the power line, causing distortion of the powerwaveform. Different loads and control devices produce different typesand degrees of distortion that may interfere with the operation of theequipment and machines that are being supplied by the distributionnetwork.

The amount of electric power used by machinery and the machinery itselfcan be affected by waveform distortions present in a power distributionsystem. Elimination or control of the distortions may provide asubstantial cost savings with respect to electrical energy consumption,and a cost savings with respect to machinery failure and repair orreplacement. Thus, mitigation and reduction of harmonic distortions inAC power distribution systems can result in a substantial energy costsavings for industrial customers.

In the context of AC power distribution systems, linear electrical loadsare load devices which, in steady state operation, present essentiallyconstant impedance to the power source throughout the cycle of theapplied voltage. An example of a linear load is an AC induction motorthat applies torque to a constant (time invariant) mechanical load.Non-linear loads are loads that draw current discontinuously or whoseimpedance varies throughout the cycle of the input AC sine wave.Examples of nonlinear loads in an industrial distribution system includearc lighting, welding machines, variable frequency drive converter powersupplies, switched-mode power supplies and induction motors that areapply torque to time-varying mechanical loads.

Harmonic currents produced by non-linear loads in an electricaldistribution system flow away from the non-linear source and toward thedistribution system power supply. The injection of harmonic currentsinto the power distribution system can cause overheating of transformersand high neutral currents in three phase, grounded four wire systems. Asharmonic currents flow through the distribution system, voltage dropsare produced for each individual harmonic, causing distortion of theapplied voltage waveform, which is applied to all loads connected to thedistribution bus.

Harmonic distortion of the voltage waveform affects AC induction motorperformance by inducing harmonic fluxes in the motor magnetic circuit.These harmonic fluxes cause heat build-up and additional losses in themotor magnetic core, which reduce power transfer efficiency. Inductiveheating effects increase generally in proportion to the square of theharmonic current. Induction motors can be damaged or degraded byharmonic current heating if the supply voltage is distorted. Negativesequence harmonic currents operate to reduce motor torque output. Thecombination of these effects reduce power transfer efficiency and cancause motors to overheat and burn out.

Harmonic fluxes in the motor windings are either positive, negative orzero sequence depending on the number or order of the harmonicdistortion that created them. Positive sequence harmonic magnetic fields(flux) will rotate in the direction of the synchronous field. Negativesequence harmonic flux will rotate in opposition to the synchronousfield, thereby reducing torque and increasing overall current demand.Zero sequence harmonic flux will not produce a rotating field, but stillwill induce additional heat in the stator windings as it flows throughthe motor magnetic circuit.

Industrial power distribution systems supply AC operating power toconnected machinery and devices that produce some harmonic distortion ofthe AC voltage waveform. Each harmonic of the fundamental frequency,depending on whether it is a positive, negative, or zero sequence, andits percentage of the fundamental, can have an adverse affect on motorperformance and temperature rise, as well as increase the energy costsof electrical service that is charged by the utility service provider.Electric utilities must generate service capacity adequate to meet theexpected peak demand, kVA (kilovolt amps apparent power), whether or notthe customer is using that current efficiently. The ratio of kW (realactive Power) to kVA (apparent power) is called the load power factor.Most utilities charge a penalty to customers when the customer's totalload power factor is low.

Apparent power can be larger than real power when non-linear loads arepresent. Non-linear loads produce harmonic currents that circulate backthrough the branch distribution transformer and into the distributionnetwork. Harmonic current adds to the RMS value of the fundamentalcurrent supplied to the load, but does not provide any useful power.Using the definition for total power factor, the real kW is essentiallythat of the fundamental (60 Hz) AC waveform only, while the RMS value ofthe apparent kVA is greater because of the presence of the harmoniccurrent components.

A low kW/kVA power factor rating can be the result of either asignificant phase difference between the voltage and current at themotor load terminals, or it can be due to a high harmonic content or adistorted/discontinuous current waveform. An unacceptable load currentphase angle difference can be expected because of the high inductiveimpedance presented by the stator windings of an induction motor. Adistorted current waveform will also be the result of an induction motorthat is applying torque to a non-linear load. When the induction motoris operating under discontinuous load conditions, or when the load isnon-linear, high harmonic currents will result, degrading motorperformance and reducing power factor.

Some power factor correction can be achieved by the addition ofcapacitors connected across the induction motor stator windings. Theresulting capacitive current is leading current that cancels the lagginginductive current flowing from the supply, thus improving the powerfactor when the induction motor is driving a linear load. For example,KVAR (Kilovolt Ampere Reactive) capacitors may be installed to correctlow power factor caused by the high inductance of stator windings.Harmonic currents produced in the load circuit or that are conductedalong the branch power distribution line from remote non-linear sourcesmay find a resonance with the KVAR capacitors, and the resulting highcurrent may cause the capacitors to fail. These harmonic currents, whencombined with the inductive reactance of the distribution network, canalso cause premature motor failure due to excessive current flow, heatbuild-up and random breaker tripping.

Controllers for reducing energy consumption of AC induction motors havebeen developed or proposed. One class of such devices uses a measure ofthe power factor of the AC induction motor to generate a feedback signalthat is used for controlling the amount of power delivered to the motor.The control signal is adjusted from time-to-time to reduce the averagepower applied to the motor during light loading in order to maintainsufficient rotor slip for operation with a relatively high power factorand good power transfer efficiency.

Various problems arise in the operation of conventional controllers,particularly when controlling power applied to non-linear loads. Forexample, complex power control factors are presented by the operation ofAC induction motors that drive pumping units (pump jacks) used to liftfluids from underground formations. Such pumping units are alternatelyloaded by a pumping rod, the weight load of the formation fluid column,and opposing counter-weights twice each pumping cycle. Moreover, twiceeach cycle the opposing loads balance and the motor is thus unloadedtwice each cycle. The constantly changing load between peak minimum andmaximum values creates severe control difficulties for power factorcontrol systems which must continuously adjust the power delivery tomaintain optimum motor efficiency and economy.

Currently, thyristor switches are in use in conventional controllers forcontrolling the AC power supplied to induction motor loads, for examplein the AC power controller disclosed in U.S. Pat. No. 6,400,119. Becauseof the fast on-off switching action (fast dv/dt) of the thyristors, highpeak voltage and high switching frequency, the input current on thesupply side of the power controller becomes distorted with highfrequency switching transients, which cause an increase of harmoniccomponents in the AC power delivered to the induction motor. Moreover,spurious noise and harmonic currents from remote sources that areconducted down the branch distribution circuit can interfere with theproper switching operation of the controller itself, resulting in lossof power control.

These factors not only reduce the power factor of the branch load, butalso interfere with motor operation and inject harmonic currents backthrough the power distribution branch and into the distribution network.Moreover, controller-generated harmonic distortion increases the RMSvalue of the load current in the power distribution branch, on which theutility service fees are based, thus increasing the customer's energycosts.

SUMMARY OF THE INVENTION

An improved power controller system is provided for increasing theoperating efficiency and performance of conventional AC induction motorsthat receive operating power from an electronic controller that employsfast switching circuits to control the application of AC power to thestator windings of the motor. The improved controller system (a)operates efficiently to drive a non-linear mechanical load under lighttorque loading as well as full-rated torque loading conditions, (b)mitigates harmonic currents from remote sources, (c) mitigatescontroller-induced harmonic currents, (d) mitigates load-inducedharmonic currents, and (e) operates compatibly with KVAR (KilovoltAmpere Reactive) capacitors that are connected across the statorwindings to improve low power factor caused by the high inductiveimpedance of the induction motor stator windings.

A primary low pass filter is connected in series between the branchphase conductors and the power controller. KVAR (Kilovolt AmpereReactive) capacitors are connected across the output terminals of thepower controller in shunt to neutral relation. The KVAR capacitor valuesare coordinated with the inductive reactance values of the statorwindings to form a secondary low pass filter across the controlleroutput terminals. The primary and secondary low pass filters isolate thepower controller and induction motor with respect to spurious noise andharmonics generated by local as well as remote sources, and alsoimproves real power transfer efficiency from the power generating sourceto the induction motor.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawing is incorporated into and forms a part of thespecification to illustrate the preferred embodiments of the presentinvention. Various advantages and features of the invention will beunderstood from the following detailed description taken with referenceto the attached drawing figures in which:

FIG. 1 is a simplified electrical circuit schematic diagram showing theinterconnection of an AC power controller for dynamically adjusting theoperating power applied to an induction motor to match non-linear loadrequirements;

FIG. 2 is a simplified schematic diagram of a non-linear loadapplication in the form of a pump jack and sucker rod pump system thatis being supplied with operating power by the power controller system ofFIG. 1;

FIG. 3 illustrates typical induction motor torque loading and sucker rodstroke displacement produced by the pump jack and sucker rod pumpingsystem of FIG. 2;

FIG. 4 illustrates typical voltage and current waveforms produced in arepresentative stator phase winding during controlled operation of theinduction motor of FIG. 2; and

FIG. 5 is a front perspective view showing the physical arrangement ofcontroller system components within a protective housing.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the invention will now be described withreference to various examples of how the invention can best be made andpracticed. Like reference numerals are used throughout the descriptionand several views of the drawing to indicate like or correspondingparts.

Referring now to FIG. 1, a conventional AC power distribution network 10supplies power from a high voltage AC power source 12 to a step-downdistribution transformer 14. The distribution transformer feeds power ata reduced voltage level to a distribution panel 16 that includesconventional three phase distribution breakers 18, 20 and 22. AC powerat a fundamental frequency of 60 Hz and 480 VAC phase-to-phase (277 VACphase-to-neutral) is conducted via a four conductor, shared neutralbranch circuit 26 containing AC phase conductors 28, 30, 32, and ashared neutral conductor 34.

Three-phase AC power is applied via the branch circuit conductors to theinput terminals N1, N2 and N3 of an electronic power controller 36. Thepower controller 36 applies controlled amounts of AC power through itsoutput terminals M1, M2 and M3 to input terminals S1, S2 and S3 of athree-phase induction motor 38. The motor 38 is mechanically coupled intorque power transfer relation to a mechanical load 40. The powercontroller 36 senses the instantaneous power demand of the mechanicalload and adjusts its power output to dynamically match the load demandrequirement during each half cycle of the applied power waveform. The ACpower applied to the AC induction motor 38 is increased and reducedautomatically as necessary to match the non-linear load demand.

Preferably, the power controller 36 is constructed as described in U.S.Pat. No. 6,400,119 entitled “Energy Conserving Motor Controller,” whichis incorporated herein by reference. As described in the specificationof that patent, first and second gate controlled switches (siliconcontrolled rectifiers) 42, 44; 46, 48; and 50, 52 are connected inparallel with each other in opposing polarity relation in each phase ofthe applied AC voltage. A trigger generator couples trigger controlsignals to the respective gates of the SCR switches in response to thetiming of sensed zero-crossing events of the AC voltage and currentwaveforms in each respective stator winding phase of the inductionmotor.

The first and second SCR switches of each phase are alternatelytriggered into a conductive state during each alternation of the appliedAC voltage and are alternately inhibited from the conductive state foran interval in time proportional to a measured difference in timebetween the AC voltage zero-crossing and the corresponding AC currentzero-crossing as determined by comparing the time difference betweensuccessive first and second interrupts corresponding to thezero-crossing events with a continuously running time base.

Referring to FIG. 4, the measured difference in time between the ACvoltage zero-crossing and the corresponding AC current zero-crossing ineach half cycle of the ΦA waveform is an indication of the instantaneousload demand. The power controller 36 senses the difference and adjustsits output to dynamically match the load demand power level during thenext half cycle of the applied AC waveform. The fast switching circuits42, 44; 46, 48; and 50, 52 in each power phase of the controller 36alternately conduct and interrupt AC power applied to the AC inductionmotor 38 in proportion to the measured difference.

By this arrangement, the power applied to the motor is increased orreduced automatically from one half cycle to the next in each phase asnecessary to match the instantaneous power requirements of the load 40.Current flow in each phase is interrupted during an interval that isproportional to the measured phase difference between the voltagewaveform and the current waveform zero crossings in the preceding halfcycle. Thus, current flow is interrupted in only one phase at a time, asthe power adjustments proceed consecutively in the three phases ΦA, ΦBand ΦC.

Harmonic currents from remote sources are mitigated by a primary lowpass filter 54 that includes three identical LC filter sections inconnected in series with the branch power distribution conductors 28,30, and 32, respectively, at the input terminals N1, N2 and N3 of thecontroller 36. The controller 36 and the induction motor 38, as well asall other components that may be connected to the filtered side of thebranch distribution circuit 26, are isolated from external noise andspurious signals generated by remote devices in other phases or otherbranches of the power distribution network 10.

Each low pass filter section includes an inductor (L1, L2, L3) connectedin series with the phase conductor and a capacitor (C1, C2, C3)connected in shunt from phase to neutral. Each LC section of the primarylow pass filter 54 has very low attenuation from DC up through thefundamental power distribution frequency (60 Hz) to a cutoff frequency(e.g., 300 Hz), and substantially attenuates all other signals above thecutoff frequency, including harmonic components up through the 11thorder and beyond.

Each section of the primary low pass filter circuit 54 preferablyincludes an inductor and a capacitor tuned to present high impedance andattenuation of signals at 300 Hz and higher, and present low impedancewith very little attenuation or loss to signals from DC through AC powerdistribution frequencies in the Hz/60 Hz range. Each section of the lowpass filter 54 provides a high-frequency attenuation ratio of 40:1 orbetter at the cutoff frequency, thus isolating the controller 36 and itsconnected components from external noise and spurious high frequencysignals.

For operation at 60 Hz AC power distribution and 300 Hz cutofffrequency, the preferred value of each capacitor C1, C2 and C3 is 3 uF,each rated at 600 VAC service and the preferred value of each inductorL1, L2 and L3 is 0.86 mH. Preferably, each inductor L1, L2 and L3 is aniron core line reactor rated at 56 amps and 40 hp, 480 VAC, 60 Hzservice. This allows the 60 Hz AC supply power to pass with virtually noattenuation, thus delivering clean, filtered three-phase AC current andvoltage at 60 Hz to the power controller 36.

According to an important feature of the invention, the clean, filteredAC current is supplied from the primary low pass filter 54 as operatingpower to the internal power supply of the power controller 36. Thisprevents interference from remote noise sources and assures stableoperation of its microprocessor, comparators, trigger circuits and othercomponents that require stable voltage levels. Moreover, because of thebilateral operation of the primary low pass filter 54, harmonics andother noise signals generated by operation of the switching componentsof the power controller 36 or by the induction motor 38 are attenuatedand suppressed, thus inhibiting injection back into the powerdistribution network 10.

The power factor of the induction motor 38 is improved and the effectsof harmonic currents generated by operation of the induction motor undernon-linear load conditions are mitigated by KVAR (Kilovolt AmpereReactive) capacitors C4, C5 and C6 that are connected across thecontroller output terminals M1, M2 and M3 in shunt to neutral. The KVARcapacitor values are selected and coordinated with the inductance valuesof the stator phase windings W1, W2 and W3 to provide a secondary lowpass LC filter sections in series between the output terminals M1, M2and M3 of the power controller and the input terminals S1, S2 and S3 ofthe induction motor.

According to an important feature of the invention, the KVAR capacitorvalues are selected and coordinated with the inductance values of thestator phase windings W1, W2 and W3 to provide a secondary low passfilter between the output terminals M1, M2 and M3 of the powercontroller 36 and the induction motor input terminals S1, S2 and S3.Each section of the secondary low pass filter 59 has very lowattenuation from DC up through the fundamental power distributionfrequency (60 Hz) to a cutoff frequency (e.g., 300 Hz or the 5thharmonic), and substantially attenuates all other signals above thecutoff frequency, including harmonic components up through the 11thorder and beyond.

The KVAR capacitors C1, C2 and C3 serve dual purposes: (1) improve thepower factor of the induction motor 38, and (2) filter current thatflows into the induction motor 38 while suppressing back-flow ofharmonic currents generated by the motor. The secondary filter 59prevents the injection of controller-generated harmonics into theinduction motor 38, and prevents the injection of inductionmotor-generated harmonic currents into controller 36 and thedistribution network 10.

Real power transfer efficiency is improved by the impedance transformingeffect of the primary low pass filter 54 and the secondary low passfilter 59. The primary low pass filter 54 transforms the power sourceimpedance, which is primarily inductive, to an effective sourceimpedance Z_(S) that functions as a balanced LC impedance within thepass band of the primary low pass filter 54. The secondary low passfilter 59 has the same effect on the highly inductive input impedance ofthe induction motor 38. The secondary low pass filter 59 transforms theinduction motor impedance into an effective load impedance Z, thatfunctions as a balanced LC impedance within the pass band of thesecondary low pass filter.

According to the maximum power transfer theorem, maximum power transfermay be achieved when the load impedance Z, is constrained to be equal tothe power source impedance Z_(S). For optimum power factor correctionand power transfer efficiency during operation of a three phase, 40 HPinduction motor at 60 Hz with 480 VAC three phase power, the preferredvalue of each KVAR capacitor C4, C5 and C6 is 5 uF, rated for 600 VACservice. Preferably, the values of the KVAR power factor correctioncapacitors C4, C5 and C6 are selected so that motor power factorimprovement, low pass filtering action and optimum power transfer areprovided.

The KVAR capacitors C4, C5 and C6 connected in combination with thestator winding inductors W1, W2 and W3 define secondary low pass filtercircuits 59. These secondary filter sections transform the highlyinductive motor load into a balanced effective load impedance Z, that iscomparable to the effective source impedance Z_(S) provided by theprimary low pass filter 54 at the input to the power controller. Carefulselection of the KVAR power factor correction capacitors C4, C5 and C6for a given induction motor will transform the load impedance presentedby the motor, thus improving power transfer in proportion to how closelythe transformed load impedance Z, matches the transformed sourceimpedance Z.

The low pass filter circuit 54, power controller 36 and the KVARcorrection capacitors C4, C5 and C6 are enclosed within a commonprotective housing 55, as shown in FIG. 5. Air cooled heat sinks (notshown) are thermally coupled to the iron core line reactors L1, L2 andL3 on the back side of the housing.

The induction motor 38 is a conventional three-phase induction motorhaving a 40 hp service rating. AC power at 60 Hz, 480 VAC line toneutral is applied to three phase stator windings W1, W2 and W3connected in a Wye winding configuration and arranged in stator slotsthat are symmetrically spaced from each other by 120 degrees. Rotarytorque is transmitted by a squirrel cage rotor R that is magneticallycoupled to a rotating magnetic flux field produced by the flow ofthree-phase alternating currents in the stator windings W1, W2 and W3.The rotor R transmits torque to an output drive shaft 58 which iscoupled to the load 40. The load 40 may be a non-linear mechanical load,for example a beam-type pumping unit 60, as shown in FIG. 2.

Referring now to FIG. 2, the power controller system 56 of the presentinvention receives AC operating power from the three phase branch powerline 26. The power controller system 56 supplies controlled amounts ofAC operating power to a beam-type pumping unit 60. The pumping unit,sometimes referred to as a pump jack, reciprocates a sucker rod 62 and adown-hole pump. The pump lifts formation fluid on each upward stroke ofthe sucker rod and oil (formation fluid) F flows into the pump on thedown stroke, produced to a well head fitting on the up stroke, and thenthe pumping cycle is repeated.

The pumping unit 60 includes a walking beam type pump jack 64, having aconventional walking beam 66 and a horse head 68. The walking beam 66 ismounted on an A-frame 70 at pivot 72. A counterweight 74 and crank arm76 are driven through a gear box 78 by the AC induction motor 38. Therotor R of the induction motor is mechanically coupled to the gear box78 by the power transmission shaft 58. A wire-line hanger 80 is attachedto horse head 68 by a short length of cable 82. The lower end of thehanger 80 is secured to the sucker rod 62. The polished section of thesucker rod 62 extends through a surface well head fitting and isconnected to a sucker rod string extending from wellhead into asubterranean reservoir through a production tubing string 86.

A conventional timer control unit 88 is connected to one phase of the480 VAC three-phase power for supplying operating power to an internalpumping cycle timer. The internal timer, which is set to match the knownreservoir fill rate, automatically enables pumping cycle operation ofthe pumping unit for a first predetermined pump-ON interval, and theninterrupts AC power to the controller 36 during a predetermined pump-OFFinterval. The timer control unit 88 includes a step-down transformerthat provides 110 VAC, 60 Hz operating power to the internal timer andcontactor relay circuits.

The timer control unit also includes circuitry for automaticallyinterrupting AC power to the controller 36 and resetting the timer tothe pump-OFF cycle in response to a pump-OFF control signal 90. Thepump-off control signal is generated in response to temporary exhaustionor depletion of formation fluid in the well bore. Hammering impact ofthe pump plunger is sensed by a conventional fluid impact sensor locatedon the wire-line hanger 80 on the upper end of the polished rod 62.Pumping action is discontinued until the reservoir replenishes the wellbore to a productive level.

Referring now to FIG. 3, waveforms 92, 94 indicate representative valuesof induction motor loading and pump stroke displacement, respectively.During normal pumping operation, the pumping unit pumps at a fixed rate,for example at 6.6 stroke cycles per minute (stroke period 9 secondspeak-to-peak). The motor torque loading 92 imposed by the sucker rodload is a complex non-linear function of time, containing positive andnegative slope ramp functions, and some ringing or oscillatingfunctions.

These torque waveform components are produced during four separateloading phases. The pump uploads formation fluid according to a positiveslope load, then transitions through zero load slope at stroke peakwhere some ringing or oscillation takes place at a relatively hightorque level as the counterweight 74 transitions through top deadcenter. The pump load then transitions along a negative load slopetoward stroke bottom. Thereafter the torque load waveform transitionsthrough zero slope at stroke bottom where some ringing or oscillationtakes place at a relatively low torque level as the counterweight 74transitions through bottom dead center.

These non-linear load fluctuations give rise to strong harmonic currentsthat may interfere with controller operation, and can be injected backthrough the power distribution branch and into the distribution network.This increases the RMS value of the load current in the powerdistribution branch, on which the utility service fees are based, thusincreasing the customer's energy costs. The power controller system 56reduces or mitigates these harmonic currents that may be caused by fastswitching action of the power controller 36 or by non-linear loads thatare powered by the power controller.

Extensive field tests have been conducted with the power controllersystem 56 installed on a working well located in Lea County, N. Mex. Thetest results are summarized in Table 1 and Table 2.

The operational data summarized in Table 1 and Table 2 were abstractedfrom logs that were recorded on separate dates two months apart inconnection with two separate tests run on the same induction motor 38and pumping unit 60. The first test was run while the motor initiallywas in need of repair and poor operating condition, with bearingproblems. The second test was run two months later on the same motorafter it had been repaired with new replacement bearings and certifiedin good operating condition. The motor 38 installed on the field testpumping unit was a 480 volt, three-phase, 40 Hp induction motor. Themotor was connected to a walking beam type pumping unit 60 that had beenin service for 15 years at the time of the field tests.

The well had been consistent in oil, water, gas production and powerconsumption for the 15 years preceding the tests. The pumping unit 60used in the field tests was located at the end of an irregular four-wirethree phase branch distribution line 26 that was subject to severespiking and power surges caused by frequent thunder storms lightningstrikes. All the branch power lines were open lines, with no insulationfrom the transformer 14 to the service pole.

The data logger used during these field tests was a Rustrak Ranger1231A. The data logger was installed to reflect readings consistent withthose of the Kilo Watt Hour meter on the service pole. The metermanufacturer was consulted for installation specifications. There wereno interruptions during the log cycles as they were recorded.

The recorded data shown in Table 1 and Table 2 reflect the performanceof the induction motor 38 with and without the controller system 56. Itshould be noted that the voltage remained consistent with and withoutthe controller system 56 installed, but the current was reduced,reactive power was reduced, the real power consumed was reduced, and themotor power factor was somewhat improved with the controller system 56installed.

The harmonic current distortions in the 3rd and 5th orders were at 5.0THD to 7.0 THD, without the controller system installed and were onaverage about 3.0 THD to 4.0 THD with the controller system installed.This demonstrates that controller system 56 was functioning effectively.It should also be noted that the current readings of the three phases ofthe induction motor were closely balanced in magnitude and stabilizedwith the controller system 56 installed.

TABLE I Test 1: 40 Hp Motor in bad condition - 4 Cycle Log DesignatedWithout Controller With Controller Line Cycle 1 Cycle 2 Cycle 3 Cycle 4Cycle 1 Cycle 2 Cycle 3 Cycle 4 Volts Line 1 495.98 498.23 497.91 501.82499.9 499.78 498.51 499.39 Volts Line 3 494.03 494.51 496.75 501.35500.3 494.51 497.81 496.78 Amps Line 1 29.48 29.37 29.39 29.89 24.724.31 24.56 24.66 Amps Line 3 29.11 29.63 29.49 29.68 24.3 23.7 24.7623.63 PF Line 1 0.359 0.352 0.348 0.388 0.347 0.322 0.386 0.347 PF Line3 0.357 0.354 0.345 0.388 0.346 0.322 0.386 0.347 Total KW 9.25 9.41 9.310.22 8.22 7.47 8.4 7.5 Measurements taken with Rustrak Ranger 1231Ameter 4 Cycle accumulated total KW without controller: 38.18 4 Cycleaccumulated total KW with controller: 31.59 Difference: 6.59 17.26%energy savings with controller.

TABLE II Test 2: 40 Hp Motor in good condition - 3 Cycle Log DesignatedWithout Controller With Controller Line Cycle 1 Cycle 2 Cycle 3 Cycle 1Cycle 2 Cycle 3 Volts Ln. 1 498.17 493.49 496.86 501.89 500.72 507.38Volts Ln. 3 495.43 494.49 498.43 503.25 501.26 498.27 Amps Ln. 1 29.0929.24 29.04 24.05 24.04 24.45 Amps Ln. 3 22.98 22.31 22.7 28.57 28.4627.29 PF Ln. 1 0.342 0.348 0.377 0.347 0.354 0.361 PF Ln. 3 0.342 0.3480.377 0.346 0.357 0.361 Total KW 8.72 8.79 9.46 7.07 7.3 7.87Measurements taken with Rustrak Ranger 1231A meter 3 Cycle Logaccumulated total KW without controller: 26.97 3 Cycle Log accumulatedtotal KW with controller: 22.24 Difference 4.73 17.53% energy savingswith controller

Although the invention has been described with reference to certainexemplary arrangements, it is to be understood that the forms of theinvention shown and described are to be treated as preferredembodiments. Various changes, substitutions and modifications can berealized without departing from the spirit and scope of the invention asdefined by the appended claims.

1. A power controller system including one or more supply inputterminals for receiving AC voltage from one or more supply phases of anAC power source, and one or more supply output terminals for conductingAC current to one or more stator phase windings of an AC inductionmotor, comprising in combination: an electronic power controllerincluding one or more power input terminals for receiving AC voltagefrom one or more of the controller system supply input terminals, one ormore power output terminals electrically connected for conducting ACcurrent to one or more of the controller system supply output terminals,and switching means coupled between the power input terminals and poweroutput terminals for controlling the conduction of current to one ormore of the system supply output terminals; a primary low pass filtercircuit including one or more input terminals coupled to one or more ofthe supply input terminals and one or more output terminals coupled toone or more power input terminals of the electronic controller; and oneor more capacitors connected in shunt phase to neutral relation acrossone or more of the controller power output terminals.
 2. A powercontroller system according to claim 1, wherein the capacitance value ofeach shunt capacitor is selected and coordinated with the inductancevalue of one or more stator phase windings of an induction motor that isto be connected to the power controller system, thereby forming incombination one or more secondary low pass filter circuits when soconnected.
 3. A power controller system according to claim 1, whereinthe capacitance value of each shunt capacitor is selected andcoordinated with the inductance value of one or more stator phasewindings of an induction motor that is to be connected to the powercontroller system, thereby transforming the effective load impedancepresented by an induction motor when so connected into an electricalimpedance that is comparable to the effective source impedance presentedon the output of the primary low pass filter circuit when the primarylow pass filter circuit is coupled to an AC power source.
 4. A powercontroller system according to claim 1, wherein each shunt capacitor israted for kilovolt ampere reactive service.
 5. A power controller systemas set forth in claim 1, wherein the primary low pass filter circuitcomprises one or more low pass LC filters connected between one or moreof the supply input terminals and one or more of the power inputterminals.
 6. A power controller system as set forth in claim 1, theelectronic power controller comprising first and second gate-controlledswitches, each switch having a respective control gate and the switchesbeing connected in parallel, opposing polarity relation with each otherbetween a first node and a second node, for each phase of the ACvoltage, wherein the first node is electrically coupled to one of thesystem supply input terminals and the second node is electricallycoupled to one of the system supply output terminals.
 7. A method ofcontrolling the application of AC operating voltage from a power sourcein one or more phases of AC voltage to one or more stator phase windingsof an AC induction motor to match the power requirements of a mechanicalload being driven by the motor, the method comprising the followingsteps performed for each phase: coupling a gate-controlled switch inseries between a selected phase of the AC voltage and a selected motorstator winding wherein the gate-controlled switch includes first andsecond control gates, one for each polarity of the AC voltage applied tothe switch and the motor; alternately triggering the gate-controlledswitch into a conductive state during each alternation of the ACvoltage; inhibiting the conduction of the gate-controlled switch duringeach alternation of the AC voltage for a time interval proportional toan interval beginning when the AC voltage of an alternation in the motorwinding passes through a first zero-crossing and ending when thecorresponding AC current of an alternation in the motor winding passesthrough a second zero-crossing; filtering the AC supply voltage that isconducted through the gate controlled switch; and connecting a capacitorin shunt phase to neutral relation across one of the controller poweroutput terminals, wherein the capacitance value of the capacitor isselected and coordinated with the inductance value of one of the statorphase windings thereby forming in combination a secondary low passfilter circuit when so connected.
 8. A method of controlling theapplication of AC operating power according to claim 7, including thestep of selecting and coordinating the capacitance value of the shuntcapacitor with the inductance value of one of the stator phase windings,thereby providing a secondary low pass filter circuit which transformsthe electrical impedance presented by the induction motor into aneffective electrical impedance that is comparable to the effectivesource impedance presented on the output of the primary low pass filtercircuit when the primary low pass filter circuit is receiving AC voltagefrom an AC power source.
 9. A method of controlling the application ofAC operating power to an AC induction motor according to claim 7,including the step of controlling the operation of the gate controlledswitch by an electronic controller, and further including the step ofapplying the filtered voltage output from the low pass filter asoperating power to the electronic controller.
 10. A method ofcontrolling the application of AC operating power to an AC inductionmotor according to claim 7, including the step of selecting thecapacitance value of the shunt capacitor in coordination with theinductance value of the stator phase winding to transform the electricalimpedance presented by the induction motor into an effective impedanceZ_(L) that is substantially in balance with the effective sourceimpedance Z_(S) presented on the output of the primary low pass filtercircuit when the primary low pass filter circuit is receiving AC voltagefrom an AC power source.